The present invention relates to a method of driving a fluorescent print head used as a light source for an optical printer (an image forming apparatus) such as a color printer, which forms an image on a recording medium such as a photosensitive film (e.g. an instant film) or a photographic paper (e. g. a silver salt paper), and to an image forming apparatus.
A fluorescent print head mounted on an optical printer such as a color printer, which uses light emitted due to electrons hitting a fluorescent substance and creates a desired image on a recording medium (e.g. a photosensitive film and a photographic paper), is well known.
FIG. 3 is a cross-sectional view partially illustrating a fluorescent print head of the above-mentioned type. FIG. 4 is a plan view illustrating luminous dots of a fluorescent print head. FIG. 5(a) is a perspective view partially illustrating the anode substrate of a fluorescent print head. FIG. 5(b) is a plan view partially illustrating the anode portion. FIG. 6 is a side view illustrating an optical printer having three fluorescent print heads for R (red), G (green) and B (blue) luminous colors.
As shown in FIG. 6, the optical printer 1 has as a dot array three fluorescent print heads 2 (2R, 2G, 2B). Each fluorescent print head 2 has luminous dots emitting R (red), G (green) or B (blue) color (or a R, G, or B filter is combined with a luminous dot of a fluorescent substance with a broad wavelength (e.g. a ZnO:Zn fluorescent substance) containing R, G and B components). A recording medium such as a film is exposed to the light beams from respective print heads 2 to form a desired image.
The three fluorescent print heads 2R, 2G, and 2B have the same structure (However, the combination of either fluorescent substances or fluorescent substances and R, G, and B filters is different). Here, the structure of a fluorescent print head 2R emitting red color light will be described below as an example.
As shown in FIG. 3, the fluorescent print head 2R has a container 6, being a box assembled with an anode substrate 3, side plates 4, and a rear substrate 5 by bonding together with a sealing glass. The inside of the container 6 is evacuated in a vacuum state.
As shown in FIG. 4, a first luminous dot column 8 of plural luminous dots 7 and a second luminous dot column 9 of plural luminous dots 7 are arranged in parallel along the longitudinal direction of the anode substrate 3 over the inner surface of the anode substrate 8. Each luminous dot 7 has an anode electrode 10 (made of a frame-like conductive thin film of aluminum), patterned on the anode substrate 10 using the sputtering and the photolithography, and a fluorescent substance layer 11 coated on the anode 10.
The fluorescent substance layer 11, for example, made of zinc oxide fluorescent substance (ZnO:Zn), or cadmium sulfide series fluorescent substance ((Zn,Cd)S:Ag,Cl), is formed in such a way that the layer 11 has an opening wider than the square opening 10a of the anode 10 and does not run off the frame. The light emitted from the surface of the fluorescent substance layer 11 radiated outside through the fluorescent substance and the anode substrate 3 from the opening of the anode 10. Hence, the area of each luminous dot 7 corresponds to the effective luminous area of the fluorescent substance layer 11 defined by the opening 10a of the anode 10.
In the first and second luminous dot columns 8 and 9, respective luminous dots 7 are led out with the anode conductor 12 and are electrically connected to the control circuit 14 on the circuit substrate 13, using, for example, TAB (tap-automated bonding, as shown in FIGS. 3 and 6.
Here, the shape of each luminous dot 7 and the arrangement of the first and second luminous dot columns 8 and 9 will be described. As shown in FIG. 4, each luminous dot 7 is in a square form of which one side has a length (a). In the first and second luminous dot columns 8 and 9, a large number of luminous dots 7 are arranged at intervals of (a) in the primary scanning direction. The luminous dots in the luminous dot column 8 and the luminous dots in the luminous dot column 9 are shifted to each other by the pitch P (=a) in the scanning direction. Moreover, the luminous dots in the luminous dot column 8 and the luminous dots in the luminous dot column 9 are spaced away from each other by the pitch (b) (an integer multiple of the pitch P in the primary scanning direction) in the secondary scanning direction. The luminous dot columns 8 and 9 also are arranged in parallel and in zigzag form.
As shown in FIG. 3, a flat control electrode 15 is arranged as a control electrode on the upper surface of the anode substrate 3. The flat control electrode 15, which is made of a conductive film (e.g. aluminum), surrounds the luminous dots 7 and anode conductors 12 and is disposed so as to be flush with the luminous dots 7. A positive voltage is always applied to the flat control electrode 15 upon drive operation to maintain the adjacent electric field at a fixed level.
In the container 6, as shown in FIG. 3, the first filament cathode 16 and the second filament cathode 17, each being a thermionic cathode, are suspended above the first and second luminous dot columns 8 and 9. The first filament cathode 16 and the second filament cathode 17 are arranged in the primary scanning direction and are spaced substantially at equal distances from the centers of the luminous dot columns 8 and 9. In the filament cathodes 16 and 17, an electron emission material is coated on an ultra-fine tungsten alloy wire (e.g. tungsten or rhenium tungsten) of a diameter of 7 xcexcm to 50 xcexcm. The electron emission material is made of a ternary oxide containing barium oxide, calcium oxide, and strontium oxide. That oxide is uniformly coated at a thickness of 5 xcexcm to 10 xcexcm over a tungsten of a diameter of several xcexcm to several tens xcexcm. The filament voltage is adjusted to set the filament cathodes 16 and 17 to 600xc2x0 C. to 700xc2x0 C. Thus each of the filament cathodes 16 and 17 functions as a thermal electron source.
A NESA film 18a, being an anti-static translucent conductive film, is formed on the inner surface of the rear substrate 5. A anti-reflection layer 18b formed of graphite is formed on the NESA film 18a. The anti-static layer 18b absorbs light from the luminous dot 7 (anode 10) to prevent it from being reflected back to the luminous dot 7. With omission of the anti-static layer 18b, the light reflected back to the light emission side leaks from the gap between the anode 10 and the flat control electrode 15. This decreases the display contrast.
Inside the enclosure 6 shown in FIG. 3, a first shield electrode 19 of a stainless steel thin plate is disposed outside the luminous dot column 8 and the first filament cathode 16. Similarly, the second shield electrode 20 of a stainless steel thin plate is disposed outside the luminous dot column 9 and the second filament cathode 17. The shield electrodes 19 and 20 are connected together to the same potential. Each of the shield electrodes 19 and 20 is a plate having a nearly L-shaped cross section, viewed from the plane perpendicular to the primary scanning direction. The flange plates are disposed in parallel on the surface of the anode substrate 3. The shield electrode 19 or 20 may be a flat plate. The flange plate of each of the shield electrode 19, 20 is disposed above the anode substrate 3 via the insulating layer 21 containing main components (e.g. a low-melting point glass) (or with the gap of about 0.5 mm or less). The shield electrodes 19 and 20 surround the filament cathodes 16 and 17 and the upper ends thereof are positioned above the filament cathodes 16 and 17. The shield electrode 19, 20 prevents the surface of the insulating layer 21 from being charged up. The shield electrode 19, 20 covers the anode conductor 12 for the luminous dot 7 and the conductor for the flat control electrode 15 to reduce the reactive current. Moreover, the reactive current passing the flat control electrode 15 and luminous dots 7 can be reduce by restricting the aperture of the opening defined by the shied electrodes 19 and 20.
In three fluorescent print heads 2R, 2G and 2B shown in FIG. 6, the luminous dot columns 8 and 9 are arranged in parallel and at predetermined intervals. The longer side of the anode substrate 3 corresponds to a horizontal direction (in the vertical orientation with respect to the paper surface) and the shorter side of the anode substrate 3 corresponds to a vertical direction (in the upper orientation of the paper surface). In the fluorescent print heads 2R, 2G and 2B, the dot-like light beam emitted from each luminous dots 7 passes through the translucent anode substrate 3 and irradiates horizontally and forward (in the right orientation on this paper). In each fluorescent print head 2R, 2G, 2B, an imaging optical system 24 formed of a prism (or a reflecting mirror) 22 and a Selfoc lens array (an equi-magnification imaging lens array) 23 is mounted on the front side of the anode substrate 3.
The imaging optical system 24 forms an erect equi-magnification image. The opening 10a of an anode 10 in the fluorescent print head 2 acts as a focal point. The photosensitive surface of the film 25 (a recording medium) acts as a projected image point 23. The imaging optical system 24 bends at a right angle the optical path of the dot-like light beam irradiated from the fluorescent print head 2 to the front side of the anode substrate 3 and guides it vertically and downward. As to the relationship between the luminous dot 7 and the photosensitive surface of the film 25 (a recording medium) in horizontal state, the longer side of the anode substrate 3 corresponds to a horizontal direction (the vertical orientation of this paper and the direction perpendicular to the shorter side of the anode substrate 3 corresponds to a vertical direction (the right orientation of this paper).
As shown in FIG. 6, the red filter R, the green filter G and the blue filter B are disposed under the Selfoc lens arrays 23, respectively. The filters R, G and B confront the film 25 so as to be spaced away from it a predetermined distance.
The three fluorescent print heads with the above-mentioned structure are mounted and modulalized as one container 27, together with the drive circuit 26. The drive circuit 26 includes the control circuit 14, mounted on the circuit substrate 13, for controlling the drive operation of various electrodes (such as anodes 10, flat control electrodes and filament cathodes 16 and 17) and the power source circuit 33.
In the recording operation of the optical printer 1 with the above-mentioned structure, the film 25 is relatively moved in the secondary direction with respect to light beams emitted from the fluorescent print heads 2R, 2G and 2B, as shown in FIG. 6. Image data decomposed into R, G and B colors are respectively sent to the corresponding fluorescent print heads 2R, 2G and 2B. The luminous dots columns 8 and 9 of each fluorescent print head 2 glow with a predetermined timing in sync with the relative movement.
In this drive operation of each fluorescent print head 2, the luminous dots 7, which is arranged in zigzag form in the luminous dot columns 8 and 9, continuously emit light beams onto the film 25 in parallel to the primary direction and in a straight line. Each fluorescent print head 2 repeatedly irradiates light beams onto the film 25 to create a desired full-color image.
However, in the optical printer 1 provided with the conventional fluorescent print heads 2 each configured of a fluorescent luminous tube, the problem is that the light amount decreases as the fluorescent luminous tube is driven and lit for a long period of time on the occasion of printing.
A decrease in light amount of the fluorescent luminous tube causes a lack of the density necessary for a recording medium (or a photographic paper). As a result, the print image quality is deteriorated.
The light amount of a fluorescent luminous tube depends on the magnitude (input: voltagexc3x97current) of a flow of electrons exciting a fluorescent substance, a luminous time period, and the luminous efficiency of a fluorescent substance. The fluorescent substance itself does not substantially change its property because the accelerating voltage is low (20 to 70 volts, 30 to 40 volts on average).
In the fluorescent print head 2, as shown in FIG. 3, filament cathodes 16 and 17 heated at high temperatures (about 700xc2x0 C.) are suspended above the fluorescent substances. The electron emission material formed of a ternary carbonate, particularly, barium (Ba) coated on the surface of the filament cathode 16, 17 is gradually evaporated during a long period of time and thus adheres to and contaminates the surface of the fluorescent substance layer 11. For tat reason, the contaminant adhered to the surface of the fluorescent substance layer 11 limits the accelerated electrons emitted from the filament cathode 16, 17 and blocks the light emission of the fluorescent semiconductor layer 11, thus decreasing the light amount.
Evaporation of the electron emission material of the filament cathode 16, 17 deteriorates the electron emission capability and decreases the electron flow, thus decreasing the light amount.
As described above, a decrease of the light amount (an initial light amount=brightnessxc3x97time) mainly is caused by the evaporation of the electron emission material (mainly Ba) of the filament cathodes 16, 17. The evaporation rate is controlled by the operational temperature of the filament cathode 16, 17. That is, as shown in FIG. 8, increasing the operational temperature of the filament cathode 16, 17 leads to a high rate of evaporation, thus accelerating a decrease in light amount. On the other hand, decreasing the operational temperature of the filament cathode 16, 17 leads to a low rate of evaporation, thus delaying a decrease in light amount.
As shown in FIG. 9, lowering the operational temperature of the filament cathode 16, 17 prolongs the operational lifetime. However, as shown in FIG. 10, excessively lowering the temperatures results in the electron emission amount (emission current) of the filament cathode 16, 17. Under the same drive conditions, the filament cathode 16, 17 moves to the temperature restriction region (the region depending on the temperature of the filament cathode 16, 17 itself) shown in FIG. 10 (vacuum tube characteristics of a thermionic cathode in case of the filament 5MG). This decreases the light amount, thus resulting in unstable light emission.
Generally, in the fluorescent print head 2 built in an image forming apparatus such as an optical printer, some output images have the spot being in luminous state at all times or the spot being in non-luminous state at all times. Particularly, the spot in non-luminous state varies its light amount because gases remaining inside the container 6 adhere to the surface of the fluorescent substance layer 11. When the light amount varies, the light amount of the luminous dot 7 previously being in non-luminous state varies at the time of outputting a different image. As a result, the density of an output image varies partially.
For that reason, conventionally, when the optical printer 1 having the fluorescent print head 2 prints an image, a variation in light amount of the non-luminous portion is alleviated. Hence, a previous light emitting operation (hereinafter, referred to as pre-light emission) is performed by light-emitting all dots, for example, for several minutes before the print operation. After stabilization of the light emission, an image printing operation is performed. In this case, the pre-light emission is preliminary light emission performed in advance to stabilize the luminous condition.
In further explanation, in the conventional structure, as shown in FIG. 7, the filament cathode 16, 17 is heated and driven at 600xc2x0 C. to 700xc2x0 C. immediately before the pre-luminous period T1. During the pre-luminous period T1, the anode voltage and the grid voltage are driven under the rated conditions (the rated condition means the same drive condition (voltage) as that in the print luminous period T2). Until the print luminous period T2 ends, the filament cathodes 16 and 17 are heated and driven while the anode voltage and the grid voltage are driven under rating conditions.
However, in the conventional pre-light emission operation, because the luminous time period not contributing to printing is added, the operational lifetime of the fluorescent print head is fastened.
The present invention is made to solve the above-mentioned problems.
An object of the invention is to provide a fluorescent print head driving method capable of improving an operational lifetime. In this method, a filament voltage is decreased during a luminous dot glowing period except a printing period, thus decreasing the evaporation amount of Ba contained in an electron emitter material for a filament cathode heated and driven, so that deterioration of the luminous efficiency is suppressed.
Another object of the invention is to provide an image forming apparatus capable of improving an operational lifetime.
In order to achieve the above-mentioned objects, an aspect of the present invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of, during the pre-luminous period, controlling at least one of an anode voltage to be applied to the anode and a grid voltage to be applied to the control electrode, to a rated voltage or less in a print luminous mode; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode.
Another aspect of the present invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of, during the pre-luminous period, controlling an anode voltage to be applied to the anode or a grid voltage to be applied to the control electrode, with a predetermined duty ratio; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode.
Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of controlling an anode voltage to be applied to the anode or a grid voltage to be applied to the control electrode, to a rated voltage or less in a print luminous mode; controlling the anode voltage or the grid voltage with a predetermined duty ratio; and controlling a filament voltage applied to the filament cathode to a rated voltage or less in a print luminous mode.
Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a blank period of a grid voltage applied to the control electrode corresponding to the blank period between glow states during which an anode voltage to be applied to the anode is written for each line of a recording medium is provided. The method comprises the steps of, during a print luminous period for which said luminous dots glow when an image is created on the recording medium, controlling the anode voltage and the grid voltage, to a rated voltage in a print luminous mode with a duty ratio corresponding to the blank period; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode.
The method according to the invention further comprises the steps of providing a blank period of the grid voltage corresponding to a blank period between glow states during which an anode voltage to be applied to the anode is written for each line of the recording medium; during a print luminous period for which the luminous dots glow when an image is created on the recording medium, controlling the anode voltage and the grid voltage to a rated voltage in a print luminous mode with a duty ratio corresponding to the blank period; and controlling the filament voltage to a rated voltage or less in a print luminous mode.
Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of controlling an anode voltage to be applied to the anode to a rated voltage or less in a print luminous mode; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode.
Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of controlling an anode voltage to be applied to the anode with a predetermined duty ratio; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode.
According to further another aspect of the invention, an image forming apparatus comprises a fluorescent print head having a filament cathode, a fluorescent substance layer having a plurality of luminous dots each emitting light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated; a controller for acquiring and outputting pre-luminous pattern data based on a pre-luminous signal and a voltage switching signal and acquiring and outputting image data and a voltage switching signal based on a print starting signal; a voltage selector for selecting the filament cathode drive voltage and the anode drive voltage to the fluorescent print head based on a voltage switching signal input from the controller; and a driver for driving and emitting the luminous dots of the fluorescent print head based on pre-luminous pattern data or image data input from the controller; whereby the luminous dots of the fluorescent print head are previously emitted based on a drive voltage input from the voltage selector and pre-luminous pattern data input from the voltage selector; whereby the luminous dots of the fluorescent print head are emitted based on a drive voltage input from the voltage selector and image data input from the driver and a desired image is created by illuminating light from the luminous dots onto a recording medium.
The apparatus further comprises a control electrode for controlling electrons emitted from the filament cathode. The voltage selector selects a drive voltage for the filament cathode, a drive voltage for the anode and a drive voltage for the control electrode of the fluorescent print head based on a voltage switching signal input from the controller.
In the apparatus according to the present invention, the voltage selector comprises plural power sources each for producing a different drive voltage and a selector circuit for selecting a power source which produces a drive voltage corresponding to a voltage switching signal input from the controller.
In the apparatus according to the present invention, the controller acquires and outputs pre-luminous drive data based on a pre-luminous signal and print drive data based on a print starting signal. The voltage selector includes a variable power source for variably producing a different drive voltage corresponding to a voltage switching signal input from the controller.